Janjutsushi Shiruru to Bisa Ghost

Usukage Yuu is almost like a ghost, with most people not being able to feel his presence at all; Yuu takes advantage of this in order to win a title in a mahjong tournament. However, there is one individual who is able to win against Yuu: a mysterious newcomer by the name of Tsukimi Shiruru. Now, as Yuu finds someone who can win against him, he begins to study under Shiruru's guidance. What is more important to win? Precise tactics or probability theory?